This invention relates to gas turbine engines and, more particularly, to engine nacelles for use therewith.
Jet engines for powering aircraft are provided with nacelles, or other streamlined structures which envelop the engine to reduce overall aerodynamic drag and improve engine performance. With the advent of large-diameter gas turbofan engines, the required nacelle structure circumscribing the fan has become increasingly heavy, thereby increasing aircraft weight and reducing its range. The problem is compounded by the fact that since the nacelle is so large and heavy it cannot be supported by the relatively lightweight, present-day gas turbine engines. It is, therefore, hung from the aircraft pylon as is the engine itself. Accordingly, there is redundancy of structure in the nacelle and engine which could be eliminated with a lightweight, integrated engine-nacelle.
Typically, in a gas turbofan engine, a fan is provided forward of a core engine, the fan being rotatably driven through shaft connection by the turbine portion of the engine. The fan serves to pass a large volume of air around the core engine thereby increasing overall engine thrust. The large volume of air which bypasses the core engine (often several times the quantity of air taken in by the core engine) is routed through an annular fan bypass duct.
The fan bypass duct is typically defined, at least in part, by the core engine and its associated housing (or core nacelle) which comprises the inner wall of the annulus. The outer wall is defined partially by engine structure, but predominantly by the fan nacelle which, as previously noted, is supported by the pylon or aircraft wing. A shroud, or ring, is provided which circumscribes a limited axial extent of the fan bypass duct, the shroud being connected through aerodynamically faired strut means to the core engine. This webbed structure is commonly known as the fan frame. In addition to the aforementioned struts, a stage of guide vanes is disposed across the annulus to remove any angular momentum from the flow exiting the fan to thereby increase axial thrust. The struts provide the load-carrying structure for the shroud while the guide vanes are loaded only in the aerodynamic sense. Integration of the struts and guide vanes would eliminate redundancy and reduce weight. The fan nacelle circumscribes the fan frame and shroud, defining the remainder of the annular fan bypass flow path and, also, the outer streamlined envelope for the engine. Redundancy exists, therefore, in both the struts and guide vanes, and in the pylon-to-engine and nacelle-to-pylon structure.
In addition, aircraft engine removals presently require the "unbuttoning" of the nacelle in order to obtain access to the engine, an often awkward procedure at best even when the nacelle is of the bifurcated variety as typified by U.S. Pat. No. 3,541,794, Johnston et al, which is assigned to the same assignee as the present invention. An integrated engine-nacelle would simplify this procedure and would enable a relatively simple engine disconnect, exterior to the engine, at the pylon. Yet another more fundamental problem has existed through non-integration of the nacelle and engine: since the responsibility for design of the various components often lies with different manufacturers, the most aerodynamically efficient matching of the two is not achieved due to overriding individual structural considerations. An integrated engine-nacelle would optimize engine efficiency, and thereby produce an added bonus to the performance improvement achievable through the aforementioned anticipated weight reduction. The problem facing the aircraft engine manufacturer, therefore, is to provide a lightweight nacelle integral with the engine structure which would improve overall performance through weight reduction and improved aerodynamic matching.